Green Infrastructure: A Primer

Eskew+Dumez+Ripple designed a rain garden capturing the rooftop rainwater in the courtyard of the Bioinnovation Center in New Orleans. Source: Eskew+Dumez+Ripple

This diagram shows how rainwater from the roof of the Bioinnovation Center in New Orleans that Eskew+Dumez+Ripple designed works. Source: Eskew+Dumez+Ripple

How can we design our infrastructure to be more adaptable, more responsive and more performative than what we currently have? Ecological systems are inherently flexible so that they can absorb natural fluctuations and perturbations in the environment. Biological diversity increases resiliency. By mimicking this flexibility and embracing flux, we can create more resilient infrastructure that can thrive in the face of change. Our sewer system, which is “grey” infrastructure, is sized for a certain theoretical storm, normally a 10-year storm, which is supposed to happen once every ten years. With climate change leading us into an unknown climate future, adaptability in our infrastructure is essential. The problem with how grey infrastructure is conventionally engineered, is that it is rigid—it works up until the point of failure. What this means, is that there is a very specific threshold in the system where it goes from functional to non-functional, which is when flooding ensues. “Green” infrastructure, which uses plants and soil to absorb water like a sponge, can help add a layer of adaptability to grey infrastructure by creating flexible flood-spaces—when dry, these can be recreational green spaces, and when wet, they serve as flood storage. By designing layers of different types of infrastructure at different scales, resiliency is being created through redundancy.

What is green infrastructure?

Green infrastructure, also known as low impact development, is the management of stormwater using ecological principles by mimicking the predevelopment hydrologic cycle. Green infrastructure merges biomimicry with engineering to create infrastructure that works with plants, microbes, insects, animals and natural processes to allow stormwater to be filtered, absorbed, infiltrated and slowly released after a storm. This is opposed to grey infrastructure, which uses highly engineered systems, such as drains, pipes and pumps to quickly convey water away from development. In the natural hydrologic cycle, water is able to infiltrate into the ground, replenishing streams, lakes and underground aquifers. In a process called evapotranspiration, plants soak up water, using the power of the sun to evaporate water through its leaves, which enables plants to pull up nutrients and water from the ground and cool the surrounding air through evaporative cooling.

Water pollution in grey versus green infrastructure. Source: Low Impact Development Manual, University of Arkansas.

When the soil becomes saturated with water, runoff is created, which is when water begins to flow across the ground. Most runoff enters drains, but green infrastructure proposes capturing this runoff and using it to feed ecological systems, such as shrubs, grasses and trees, that filter the water as well as reduce the amount of water entering the sewer system. Runoff can be channeled into vegetated ditches called bioswales, and into water-loving gardens called rain gardens. Curb cuts on the street can open up channels for water to feed tree pits. Green roofs can help capture and reuse water, creating habitat while also better insulating buildings and protecting roof membranes.

Why use green infrastructure?

Human development changes the natural hydrologic cycle through the creation of impervious surfaces such as concrete, asphalt, roofs, sidewalks, parking lots, roads and highways that do not let water infiltrate, or absorb, into the ground. Instead, water is conveyed over these surfaces as runoff into drains, which empty into sewers, and then our local waterways. Along the way, the runoff picks up pollutants such as dirt, trash, heavy metals, oils, greases, animal waste, pesticides, and fertilizers. The water can also become warm, picking up heat from the sun-baked roads. All of these elements can create polluted water that causes it to become less hospitable to life, killing off biodiversity and even effecting larger fisheries and ecosystems downstream. All of the stormwater that touches the road normally ends up in our local waterways, polluting the waters we drink, swim, fish and play in. The U.S. Environmental Protection Agency (EPA) claims that only 16% of the watersheds within their national study had good water quality (U.S. EPA, The Index of Watershed Indicators, 1997). Runoff from agriculture, construction sites, industries, lawns, and roads all seriously degrade water quality. Grey infrastructure simply transports the pollution without ever treating it.

Plants and soils filter, infiltrate and evapotranspirate water. Source: Low Impact Development Manual, University of Arkansas.

Grey infrastructure also helps create flash floods by quickly conveying large amounts of runoff into streams, leading to the movement of large amount of water in a short amount of time. Green infrastructure helps reduce peak flooding by reducing the amount of runoff created. Water is slowly released or reused rather than all being conveyed into the sewers at once. These flash floods also lead to stream scouring, which is when high velocity water erodes the sides of stream beds, making it difficult for fish to breed, destroys vegetation, erodes habitat through changing the natural flow dynamics, can add high amounts of suspended solids to the water impacting water quality and aquatic diversity, and creates unstable channel banks susceptible to collapse.

The runoff volume is larger in urbanized areas leading to flash floods and stream scouring. Source: Low Impact Development Manual, University of Arkansas.

“Research indicates that when impervious area in a watershed reached 10 percent, stream ecosystems begin to show evidence of degradation, and coverage more than 30 percent is associated with severe, practically irreversible degradation.” (Metro Portland, Green Streets: Innovative Solutions for Stormwater and Stream Crossings).

Urban stream syndrome is a characteristic seen in most urban streams and is characterized by low stream flows followed by periodic flash floods, altered stream morphologies, elevated nutrient and contaminant levels, excessive sedimentation, loss of species diversity and higher water temperatures.

Additionally, green infrastructure can help prevent combined sewer overflows, which in older cities is when the stormwater and sewer system are connected. During large rainfalls, the sewage treatment plants get backed up with the large volumes of water, and raw sewage is dumped into the local waterways.

Problems green infrastructure can help ameliorate: flash flooding from large amounts of impervious surfaces, water pollution from street runoff, and erosion of streams from high runoff velocity. Source: Low Impact Development Manual, University of Arkansas.

The Keller Library and Community Center in New Orleans has a rain garden designed by Eskew+Dumez+Ripple and Spackman Mossop & Michaels. The design includes a wetland garden, bioswale and detention area. Source: Eskew+Dumez+Ripple

Multifunctional Infrastructure

Beyond green infrastructure’s primary functions of improved water quality and reduced flooding, there are many ancillary benefits that create added value once built. Rather than creating uni-functional infrastructure that only serves one purpose, like a drain and pipe, green infrastructure creates multifunctional infrastructure by providing multiple functions and benefits. In cash-strapped cities, this could potentially allow for multiple funding streams and grants to build and maintain infrastructure. There is a better return-on-investment due to multiple, rather than singular, added values to the projects.

Green infrastructure provides many, if not all of the below ecosystem services, especially if the project incorporates these goals from the inception of the design:

Atmospheric regulation

Climate regulation

Disturbance regulation

Water regulation

Water supply

Erosion control and sediment retention

Soil formation

Nutrient cycling

Waste treatment

Pollination

Species control

Refugia/habitat

Food production

Raw material production

Genetic resources

Recreation

Cultural enrichment

Some added environmental benefits that green infrastructure can create are:

Reduced and delayed stormwater runoff volume

Reduced localized flooding

Water supply retention

Improved air quality by plants helping to filter the air

Reduced urban heat island effect by providing shading, blocking the sun from reaching heat-absorbing materials such as concrete, and evapotranspiration cooling the air. This can lead to less energy use in buildings.

Additionally, green infrastructure can beautify communities through adding green spaces and inviting wildlife into our world. There are added benefits of community livability such as:

Improved aesthetics

Increased recreational amenities

Reduced noise pollution

Improved air quality

Improved corridors encourage people to walk or bike

Improved human health

Improved outdoor thermal comfort. Trees help with direct shade to houses and to pedestrians. They store less heat and return heat back to the atmosphere faster than concrete so they are less of a heat sink, and they cool down the surrounding air by transpiration.

The most at-risk communities such as the elderly, poor and the homeless don’t operate air conditioners, have low tree-canopy, and could benefit from trees. (Source: “Census Tract-Level Outdoor Human Thermal Comfort Modelling and Heat-Related Morbidity Analysis During Extreme Heat Events in Toronto: The Impact of Design Modifications to the Urban Landscape.”)

Improved social cohesion, the encouragement of social interactions and use of public space